Isomers | Properties of carbon | Biology | Khan Academy
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Isomers | Properties of carbon | Biology | Khan Academy

– Many times in chemistry
we’ll see different molecules that have the same constituent atoms. For example, these two molecules here, they both have four carbons. One, two, three, four. One, two, three, four. So if I were to write
their chemical formula, it would be C4 and then they both have, one, two, three, four, five,
six, seven, eight, nine, ten. One, two, three, four, five,
six, seven, eight, nine, ten hydrogens. So both of them, both of them
have the chemical formula C4H10. C4H10, but they’re still
fundamentally different molecules and you can see that because
they have different bonding. For example, over here we
have a carbon that is bonded to three other carbons and a hydrogen. Over here I can’t find
any carbon that’s bonded to three other carbons. I can find ones that are
bonded to two other carbons, but not one that’s bonded
to three other carbons. So, how we’ve put the atoms
together, is actually different. They’re bonded to different things. And so when we have the
situation where you have the same constituent atoms,
where you have the same chemical formula, but
you’re still dealing with different molecules
because either how their bonds are made or what their shape is, we call those isomers. So an isomer, isomer, you have the same chemical formula, same chemical formula. But you could have different
bonding but different, different bonding, bonding or shape, bonding, shape or orientation. Orientation. So over here you have
just different bonding and this type of isomer is
called a structural isomer. So these characters
are structural isomers, same constituent atoms,
but different bonding. Structural isomers. So that’s structural
isomers right over there. Now when you look at this pair or this pair, you’ll say those don’t look
like structural isomers. Not only do they have
the same constituents, both of these for example
have four carbons, four carbons and they both have one, two, three, four, five, six, one, two, three, four,
five, six, seven, eight, and they both have eight hydrogens. So these are both C4H8, it’s looks like they’re bonded similarly. For example, I mean the
left hand side here, these look identical and one the right hand side, you have a carbon bonded to another carbon that’s bonded to three hydrogens, carbon bonded to another carbon that’s bonded to three hydrogens. Carbon bonded to a hydrogen, carbon bonded to a hydrogen, so it looks like the
structure of the bonding, everything’s bonded to the same things, but you might notice a difference. Over here, on the right hand side, this CH3 is on the bottom right, while over here it’s on the top right and you might say okay well we know, what’s the big deal there, these, you know, all these molecules, they’re all moving around, maybe they’re rotating
with respect to each other and these things could, this thing could have rotated down to become what we have up here. If this was a single bond. A single bond would allow
for that type of rotation, it would allow for these things
to rotate around each other. For the molecule to
rotate around that bond, but a double bond does
not allow that rotation. So this fixes these two things, this fixes these two things in place. And because of that, these are actually two different molecules. Over here on the top,
you have the CH3 groups, they’re both, they’re both,
I guess you could say, facing down or their both on the same side of the double bond, while over here they’re on different sides of the double bond and so this type of isomerism, where you have the same constituents and you even have the same bonding, this is called stereoisomerism. So over here we’re caring
much more about how things sit in three dimensions. We don’t just care about
what’s bonded to what or the constituents and actually this one is, as we’ll see, is also a stereoisomer because this carbon is bonded to the same
things in either case. So these are both, these
are both situations, there are both stereoisomers, stereoisomers, and this particular
variation of stereoisomer is called a cis trans isomer. Cis is when you have the
two groups on the same side, cis, and trans is when you have the two groups on the opposite sides of the double bond. Cis trans isomers. Cis trans isomers. Isomers, and these are often
called geometric isomers. Geometric, geometric isomers. So that’s a subset, so when
I’m talking about cis trans or geometric, I’m talking about these two characters over here. They are a subset of the stereoisomers. Now what’s going on over here? I have no double bond, I’m not
talking about cis and trans. The carbon, as I’ve just said, is bonded to fluorine, chlorine,
bromine, and a hydrogen, fluorine, chlorine,
bromine, and a hydrogen. How are these two things different? And the way that they’re different is if you were to actually
try to superimpose them on each other. You will see that it is impossible. There are mirror images of each other and because there’s four
different constituents here, you can actually not
superimpose this molecule onto this molecule over here and actually because of that, they actually have different
chemical properties, and so this over here,
these two characters, which is a subset of stereoisomers. Stereoisomers are
concerned with how things are positioned in three dimensions, not just how their bonding is different, but this subset where you
have these mirror images that cannot be superimposed, we call these enantiomers. So these two characters, these are enantiomers. Enantiomers, and enantio comes from Greek, the Greek word or the Greek root opposite. So these are opposites of each other, they cannot be superimposed, they’re mirror, they’re mirror images. So all of these are different
variations of isomers and once again, you might say, okay theses are clearly
two different molecules that have different bonding, but even cis trans isomer will have different chemical properties. These two in particular,
they aren’t that different but they do have different
chemical properties, but sometimes they’re so different that one might be able to
exist in a biological system while the other is not. One might be okay for your health, and the other might not
be okay for your health. Same thing for enantiomers. One might be biologically
active in a certain way and the other one might
not be biologically active in that same way.


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